CHOICE OF A BEARING
Loads and dimensions are the most important factors during the choice of a bearing. These two components are, in the majority of cases, bound to the machinery's project: the choice is therefore restricted to limited cases. Rules or exact tables do not exit usually a ball bearing bears high speeds and low loads, whereas a roller bearing stands lower speeds and higher loads.
Speed limits of a bearing are determined by working temperature. During the choice of it, the following rules have to be followed:
- Gravity of operation
- Machinery's duration
- Encumbrance limit of the bearing
- Negative factors, such as: vibration, collision, heat, dirt,
acceptable noise and so on.
SELECTION OF BEARING SIZE.
The size of a bearing is selected by considering different factors, such as its supposed operational life, loads to which it is subjected and prescribed operating safety.
BASIC LOAD RATINGS
To calculate bearings dimensions, the basic dynamic load rating "C" is used; this factor expresses the admissable load suitable to give a basic rating life up to 1.000.000 revolutions.
Basic dynamic and static loads rating for ISB® bearings have been determined in accordance with standard ISO 281.
Considering the basic dynamic load rating, is calculated the service time until the fatigue of the materials appears, determining in this way the calculated rating life. In the case of low speeds, low oscillating movements or stationary applications, " C0" basic static load rating is considered.
The basic static load rating is defined as the load acting on the stationary bearing. It corresponds to a calculated contact stress between the most heavily loaded rolling elements and the raceway of:
- 4600 N/mm2 for self-aligning ball bearings.
- 4200 N/mm2 for all other ball bearings.
- 4000 N/mm2 for all roller bearings.
This stress produces a permanent deformation of both rolling elements and raceway, deformation which is about 1/10000 (0,0001 dw) of the rolling element diameter. Loads are pure radial for radial bearings and pure axial for thrust bearings.
STATIC LOAD RATING BEARINGS
When the bearing is stationary or rotates at very slow movements or speeds (lower than 10 r/min), basic static load is not determined by the material fatigue but by permanent deformation caused at the rolling elements and the raceway contact.
The same rule is used for rotating bearings standing heavy shock loads which act during a fraction of their revolution. Generally, the value of the load may increase up to the value of the basic static load C0, without altering the bearing operation properties. Combined static load (radial and axial
load acting together on bearing) must be converted in equivalent static bearing load.
This is defined as the load (radial for radial bearings and axial for thrust bearings), which if applied would cause the same permanent deformation as the real load operative upon the bearing:
P0 = X0 Fr + Y0 Fa
P0 is the equivalent static bearing load, N;
Fr is the radial component of the heaviest static
Fa is the axial component of the heaviest static
X0 is the radial load factor of the bearing;
Y0 is the axial load factor of the bearing.
The life of a bearing can be considered as the number of the revolutions or the number of operating hours, that the bearing is able to endure before the first sign of fatigue appears on one of its ring, on the raceway, on the rolling elements. If we want to consider only the fatigue on the bearing operating surfaces, the following conditions have to be observed:
- Forces and loads considered when evaluating the bearing, should correspond to the real operating conditions.
- Proper lubrication should be assured during the entire operative period.
- Experience shows that the failure of many bearings does not depend only on fatigue, there are other factors besides this such as: selection of an inadequate bearing type, improper operation or lubrication, outer particles in bearings etc.
BASIC RATING LIFE
The basic rating life of a single bearing or of a group of identical bearings operating under the same conditions, is the life corresponding to a reliability of 90%. The average life of a group of bearings is higher than the basic rating life. Basic rating life is marked with L10 (millions of revolutions) or with L10h (operating hours). L10 can be calculate using the equation
L10 is the basic rating life, in millions of revolution;
C is the basic dynamic load, N;
P is the equivalent dynamic bearing load, N;
p is the exponent of the life equation with the following values;
P=3 for ball bearings;
P=10/3 for roller bearings;
For bearings operating hours, can be calculated using the equative
L10h =(1.000.000 / n · 60) · (C / P)p
n is the rotation speed, expresses in rotations per minutes.
Values of the basic rating life L10 (millions of revolutions) as a function of the ratio C/P, can be found in table 2. Values of the basic rating life L10h (operating hours) as a function of the ratio C/P and speed can be found in table 2.2 for ball bearings nd 2.3 for roller bearing. To determinate bearing size it is necessary to base the calculations on the rating life corresponding to the purpose of operation. Usually this depends on type of machinery, on the service life and on the requirements regarding operational safety.
The speed limit can be defined as the maximum rotation speed reached by a bearing, without compromising its performances. The speed limit depends on different factors, such as: type of bearing, magnitude of load, tolerance class, cage design, operational clearance, lubricant, lubrication and cooling conditions, and so on. In case of oil lubrication, the bearing speed
limit can be approximately determined for radial bearings, as a function of the mean bearing diameter; concerning thrust bearings, instead, speed limit is determined as a function of
the dimension and weight of the bearing's counting. In bearing tables, the value of speed limit are given for both grease and oil lubrication. In the case of insufficient informations about quality of lubrication and operating conditions, the effective speed is recommended not to exceed the 75% of the speed indicated in this catalogue. In the case of heavy loads, when the rating life is shorter than 75000 operating hours and bearing mean diameter is larger than 1000 mm, speed limits indicated in this catalogue, have to be multiplied by factor "f". If the load is combined, the speed from this catalogue have to be multiplied by factor "f1".
SPECIAL CASE LOW SPEEDS
At very low speeds it is impossible the formation of an elastohydrodinamic lubricant film between the rolling elements and raceway. Lubricants with special additives should be used in such cases.
If in long term stationary conditions, bearings start vibrating,
the micromovement at the rolling elements and at the
raceway contacts produces damages on the contact surfaces.
In this way vibration level is increased and the life is shorter. Oil
lubrication is preferable to grease lubrication.